Using a conversion process from electric signals to acoustic waves for a piezoelectric material, the film acoustic wave device functions as a filter or a resonator.
FIGS. 34, 35, 36 and 37 are examples of conventional film acoustic wave devices of this type as disclosed in the Japanese examined patent publication “sho61-269410” (hereinafter document 1).
FIG. 34 is a configuration of the conventional bulk acoustic wave device of this type.
FIG. 35 is a cross-section cut through A—A of FIG. 34.
A description of the numbered components indicated in the figures follows: a glass substrate 1; a piezoelectric thin film 2 made of zinc oxide (ZnO); an interdigital transducer of input side 3; and an interdigital transducer of output side 4; electrode fingers 5; and opposing electrodes 6 made of aluminum (Al).
FIGS. 36 and 37 are graphs that show properties of this type of conventional film acoustic wave device of FIGS. 34 and 35. FIG. 36 shows a relationship between an acoustic velocity Vs and a normalized thickness of thin film kh. FIG. 37 shows a relationship of an electromechanical coupling constant K2 and the normalized thickness of thin film.
FIGS. 38, 39 and 40 are examples of the conventional film acoustic wave devices of this type as disclosed in the Japanese unexamined patent publication “sho63-18708” (hereinafter document 2).
FIGS. 38 is a cross-section similar to FIG. 35.
FIG. 39 shows a relationship between the acoustic velocity Vs and the normalized thickness of thin film for the conventional film acoustic wave device of FIG. 38. FIG. 40 shows a relationship of the electromechanical coupling constant K2 and the normalized thickness of thin film.
FIGS. 41 and 42 are examples of the conventional acoustic wave devices of this type as disclosed in the Japanese unexamined patent publication “hei2-189011” (hereinafter document 3).
A description of the numbered components indicated in the figures follows: the electrode fingers 5; and a piezoelectric substrate 7.
An operation of the conventional film acoustic wave device is described using FIGS. 34 to 42.
In FIGS. 34 and 35, the electrode fingers 5 are placed on top of the glass substrate 1, and then the piezoelectric thin film 2 made of ZnO is placed on top of the two. An electric field is formed on an intersecting part of the electrode fingers 5 from electric signals applied to the interdigital transducer of input side 3. Due to the electric field, the piezoelectric thin film 2 is stretched to excite acoustic waves. The acoustic waves that have been excited at the interdigital transducers of input side 3 propagate in a direction parallel to a surface, and reaches the interdigital transducers of output side 4 accompanied by the electric field and the acoustic vibrations. At the interdigital transducers of output side 4, the electrode fingers 5 again receive the electric field which is formed by the acoustic waves, and change the electric field back to the electric signal. Since a reverse conversion of electric signals and acoustic waves is possible, the process of reverse conversion of the electric field made by the acoustic waves back to the electric signals is considered same as the case of the interdigital transducers of input side 3.
There are a number of modes for the propagation of acoustic waves through the piezoelectric thin film 2 as shown in FIG. 35. Example of the modes are: surface acoustic waves which propagate in the direction parallel to the surface due to a concentration of energy at the surface; bulk waves which propagate in the direction parallel to the surface; and the bulk waves which propagates in a direction of thickness. For any of these modes, intensities of acoustic wave excitations are determined by materials being used, combination of the materials, the physical dimensions such as thickness of each material, as well as the configuration of electrodes that excites the acoustic waves. The film acoustic wave device of FIG. 35 uses the surface acoustic waves. The configuration of electrode fingers 5 as shown in FIGS. 34 and 35 are commonly being used to excite the surface acoustic waves
The efficiency of conversion from the electric signals which are applied to the interdigital transducers of input side 3 to the surface acoustic waves relates largely to the performance of the film acoustic wave device, and as one indicia that shows such conversion efficiency, there is the electromechanical coupling constant K2. The larger the electromechanical coupling constant K2, for example, filters that are less damaging and have wide-ranging properties become possible. The electromechanical coupling constant K2 is determined by the materials being used, the combination of the materials, the physical dimensions such as the thickness of each material, and the configuration of the electrode(s) that excites the acoustic waves.
The conventional film acoustic wave device of this type in document 1 uses PbO—B2O3 glass with a density of ρ=5.7±0.3, Lamé's constant μ=(0.48±0.02)×1011 N/m2, Poisson's ratio σ=0.25 as the glass substrate 1, as the electrode fingers 5 made of aluminum, and the piezoelectric thin film 2 made of ZnO. Thicknesses are: 0.1 μm for the electrode fingers 5; 0.3˜25.5 μm for the piezoelectric thin film 2; and 0.1 μm for the opposing electrodes 6. FIGS. 36 and 37 illustrate the properties of the film acoustic wave devices with this configuration, as described in the document 1.
FIG. 36 is a graph that shows a relationship between the acoustic velocity Vs and the normalized thickness of thin film kh. FIG. 37 is a graph that shows a relationship between the electromechanical coupling constant K2 and the normalized thickness of thin film kh.
In this content, h refers to a thickness of piezoelectric thin film 2, and k refers to a wave number of the surface acoustic waves that propagate in the direction parallel to the surface. The normalized thickness of thin film kh is a multiple of the wave number k and the thickness h. Given that a wavelength of the acoustic wave is λ, and a frequency is f, the wave number k is (2π/λ) or (2πf/Vs), so under a fixed frequency f the wave number k is also a fixed number that the normalized thickness kh on a horizontal axis is possible to be substituted with the thickness h. That is, under a fixed frequency f, FIG. 36 is indicating a relationship of the acoustic velocity Vs and the thickness h of piezoelectric thin film 2, and even when the thickness h changes, the acoustic velocity Vs is fixed. Likewise, for a fixed frequency f, FIG. 37 is showing the relationship of the thickness h of piezoelectric thin film 2 and the electromechanical coupling constant K2, and in a range of kh from 3 to 4, the electromechanical coupling constant K2 is close to a maximum, indicating that it is also fixed.
Accordingly, by selecting materials of glass substrate 1, etc. as described previously, even if the thickness of piezoelectric thin film 2 varied, the acoustic velocity Vs and the electromechanical coupling constant K2 for the film acoustic wave devices are almost fixed. The acoustic velocity Vs relates to a center frequency of the film acoustic wave device, and the electromechanical coupling constant K2 largely relates to an insertion loss of the film acoustic wave device. Thus, within a range of the frequency f and the thickness h of piezoelectric thin film 2, the range of normalized thickness of the thin films kh is from 3 to 4 as in FIGS. 36 and 37, and the center frequency and the insertion loss of film acoustic wave device is approximately a fixed number.
FIG. 38 is showing the conventional film acoustic wave device of this type as in the document 2, and is a cross-sectional view similar to FIG. 35.
A description of the numbered components indicated in the figure follows: the glass substrate 1, the piezoelectric thin film 2 made of ZnO or aluminum nitride (AlN), and the electrode fingers 5 that make up the interdigital transducers.
Similar to FIGS. 34 and 35, the conventional film acoustic wave device of this type shown in FIG. 38 is using the surface acoustic waves. The configuration resembles the configuration shown in FIG. 35 where the electrode fingers 5 are placed on top of the glass substrate 1, and then place the piezoelectric thin film 2 on top of the two. However, in the example of FIG. 38, the opposing electrodes 6 are not placed on top of the piezoelectric thin film 2. The fact that the surface acoustic waves are excited by the electric field formed at the intersecting electrode fingers 5 is same as in FIGS. 34 and 35, but because the surface of piezoelectric thin film 2 has no metal on its surface, the film acoustic wave device of FIG. 38 has different properties from the example illustrated in FIGS. 34 and 35.
FIG. 39 shows a relationship of the normalized thickness of thin film kh and the acoustic velocity Vs. FIG. 40 shows a relationship of the normalized thickness of thin film kh and the electromechanical coupling constant K2.
Although the materials being used and the configuration are similar to those of FIGS. 34 and 35, a reason for the film acoustic wave device illustrated in FIG. 38 being so different in properties from FIGS. 36 and 37 is the non-metallic surface of the piezoelectric thin film 2. A case illustrated in FIG. 39 is different from the case illustrated in FIG. 36, where the acoustic velocity Vs changes when the normalized thickness of thin film kh changed. On the other hand, at a region of the normalized thickness of thin film kh greater than 2, the electromechanical coupling constant K2 becomes greater than 2. Therefore, when the thickness h of the piezoelectric thin film 2 is changed at the region of normalized thickness of thin film kh greater than 2, the acoustic velocity Vs changes but the electromechanical coupling constant K2 does not change a great deal. This means, the center frequency of the film acoustic wave device is adjusted by directly changing the thickness h of piezoelectric thin film 2. In document 2, as methods of adjusting the thickness h of piezoelectric thin film 2, for example, illustrates the use of a sputter to make a thicker film and a use of an etching method to make a thinner film. As long as the configuration is like those illustrated in FIG. 38, there will be no effect on the electrode fingers 5 by changing the thickness of piezoelectric thin film 2 using the etching or sputtering methods.
FIGS. 41 and 42 illustrate the conventional acoustic wave devices of this type as disclosed in document 3.
For those cases, the piezoelectric substrate 7 is used instead of the piezoelectric thin film 2. A numeral 5 is indicating the electrode fingers 5.
The conventional acoustic wave device of the type shown in FIG. 41 is the surface acoustic wave device used by exciting the surface acoustic waves by the electrode fingers 5. The velocity of surface acoustic waves that propagate through the electrode fingers 5 is known to have a different acoustic velocity from the acoustic velocity at a region where there is no electrode fingers 5, and this is due to the effects of a mass load and an electrical boundary condition of the electrode fingers 5. For the surface acoustic wave device of FIG. 41, by changing the thickness of electrode fingers 5 by etching the electrode material, the acoustic velocity is changed by the mass load effect, and then the center frequency of the surface acoustic wave device is adjusted. For a detailed description on the effects of change in frequency from the mass load effect, refer to “Journal of Electronics, Information and Communication Engineers of Japan A, Vol. J74-A, No. 9, pp. 1359˜1365, September 1991” (hereinafter document 4).
For the conventional acoustic wave device of this type shown in FIG. 42, parts of the piezoelectric substrate 7 are scraped off where there is no electrode fingers 5 using the etching method, to adjust the center frequency. With such a structure where the surface of piezoelectric substrate 7 has been scraped off as in FIG. 42, in are as of different surface levels of the piezoelectric substrate 7, a delay is known to arise from an influence of stored energy on the surface acoustic waves that propagate through the different surfaces of piezoelectric substrate 7. The varied surface levels allows for an equivalent adjustment of the center frequency of the surface acoustic wave device. For a detailed description of the adjustment of center frequency from the etching method the surface of piezoelectric substrate 7, refer to “IEEE Transactions on Sonics and Ultrasonics, Vol. SU-29, No. 6, pp. 299˜310, November 1982” (hereinafter document 5).
The case of forming the piezoelectric thin film 2 and a metal electrode is described using FIGS. 43 and 44.
Standard processes of forming the piezoelectric thin film 2 and the metal electrode(s) are sputtering and vacuum evaporation. In these methods of forming the films, looking from a target 8 of the sputter and vacuum evaporation, at a central portion of the wafer, the resulting film becomes relatively thick, and at a periphery of the wafer the resulting film becomes relatively thin. For example, as in FIG. 43 when the target 8 and the wafer 9 where the film components land are arranged one-to-one inside a vacuum container 10, then the formation of film in the central portion of wafer 9 is has a thickness of hc and the periphery of wafer 9 has a thickness of he, as shown in FIG. 44. Therefore, in this type of film acoustic wave device, the adjustment of frequency is needed to overcome the variation in the thickness of piezoelectric thin film and film formed on the metal electrode.